Active spells should use real time instead of timestamps

Just cast any sustained spell to yourself and try to change the time via console:

set gamehour to X

or

set timescale to X

In OpenMW such changes affect real spell duration since we determine it using a spell casting timestamp, current timestamp and timescale:

end = start + effectDuration * timeScale / (60*60);

When the timescale is 0, spellcasting does not work at all.

Morrowind uses the real time do determine the spell time left, so if the spell should work 5 real seconds, it will work 5 real seconds.

So the suggested solution is to use the real time during spell update, but I am not sure what to do with spells from savegames since we store only spell casting timestamp.